A transfer case for a vehicle typically operates in two different gear ratios, often referred to as a high range and a low range. The vehicle controller must identify the current operating gear ratio, i.e., the current operating range, in order to provide the correct engine and transmission controls management, and to meet onboard diagnostic requirements. The vehicle controller must quickly and accurately identify the current operating range of the transfer case in order to prevent damage to other components of the vehicle. For example, when the vehicle controller identifies that the transfer case is operating in the low gear ratio range, the vehicle controller may, for example, limit the vehicle speed to protect a planetary gearset of the transfer case, limit engine torque to protect drive shafts and axles, provide the appropriate engine pedal progression map for vehicle performance, provide the correct transmission output shaft speed calculation to provide proper transmission shift schedules, and allow for the proper operation of the onboard diagnostic monitors.
Typically, multiple algorithms and multiple calibrations for the same algorithm are required to verify the transfer case ratio. This is due to differences in vehicle configuration such as the transmission type; whether an automatic or a manual transmission is used; such as the number of transmission gears and their gear ratios; and the location and the number of speed sensors. Some vehicles have only an engine speed sensor or a transmission input shaft speed sensor. The most advantageous sensor for verifying the transfer case ratio is the transmission output shaft speed sensor, along with the transfer case output shaft speed sensor. Most vehicles with automatic transmissions are now equipped with the transmission output shaft speed sensor but this sensor may be removed on some applications to package the transfer case in the driveline.
Most algorithms that are required to use an engine speed sensor or the transmission input shaft speed sensor to verify the transfer case ratio will compare a calculated transfer case ratio to the theoretical transfer case ratio. This is accomplished by inferring the value of the transmission output shaft speed and then dividing by the transfer case output shaft speed. The transmission output shaft speed is inferred by multiplying the engine speed or transmission input shaft speed by the theoretical ratio of the transmission current commanded gear. When the transmission input shaft speed is noisy or erratic or when the clutches in the transmission are slipping in a particular gear, the inferred value of the transmission output shaft speed is usually incorrect. This often results in a false verification of the transfer case ratio.
Inferring the transmission output shaft speed requires the transmission to remain in the same gear long enough to verify the transfer case ratio. Requiring the transmission remain in a particular gear for a minimum time improves the robustness of the transfer case ratio verification. However, typically the driver will quickly traverses through the lower transmission gear ratios, particularly on manual transmissions, which prolongs the verification of the transfer case ratio. An additional problem occurs when an automatic transmission has an overrunning clutch in a particular gear. If the overrunning clutch is present, the vehicle must remain in a power-on (engine driving the transmission) condition in order for the transmission gear ratio to equal the theoretical transmission gear ratio. During a power-off (wheels driving the engine) condition, the transmission gear ratio is no longer equal to the theoretical gear ratio—due to the overrunning clutch—which causes the inferred transmission output shaft speed calculation to be incorrect.
Verifying the transfer case ratio appears to be a simple process. However, multiple algorithms and calibrations are required when confronted with multiple vehicle configurations, varying number and location of speed sensors, vehicle failures such as noisy speed signals, slipping clutches—on both manual and automatic transmissions—variation in vehicle driving patterns, difference in gear ratios etc. Even if the vehicle has a transmission output shaft speed, which eliminates the requirement of inferring the transmission output shaft speed, the vehicle should account for a failed sensor.